Critics of the U.S. National Security Agency's U.S. telephone records program are missing the point when they say the agency can't point to one case where the collection was critical to preventing terrorism, two people formerly involved with the program said.

The phone records collection program may not be the only tool that stops terrorist plots, but it's an important tool that helps lead investigators to terrorists, said Steven Bradbury, who was head of the Office of Legal Counsel at the U.S. Department of Justice under former U.S. President George W. Bush.

Many critics have questioned the effectiveness of the phone records program, with the U.S. Privacy and Civil Liberties Oversight Board (PCLOB) last week taking the position that the program is both illegal and ineffective. But the program may help stop the next 9/11 attack on the U.S., Bradbury said Tuesday at the State of the 'Net conference in Washington, D.C.

The phone records collection program is "necessary to protect the nation from foreign attack," he said. "The question of the necessity of the program is not fairly measured simply by whether the use of this database has been the one final piece of evidence that stopped the bomb from going off at the last minute. It's a question about whether it's been an input in counterterrorism investigations."

Bradbury and James Carr, a former U.S. Foreign Intelligence Surveillance Court judge, both defended the phone records program during a discussion about reforming the NSA. Carr has proposed that FISC judges be able to seek an opposing view when DOJ lawyers ask for surveillance orders, but he also agreed that the phone program can help investigators connect the dots in terrorism investigations.

The program provides "precursor information," Carr said.

Others at the conference called for the U.S. Congress to abolish the phone records program. Senator Rand Paul, a Kentucky Republican, said he plans to file a class-action lawsuit against the NSA as soon as next week. A website for the suit, Defendthe4th.com, has about 350,000 people who've signed on as plaintiffs, but Paul said he'd like to see millions more signatures.

The collection of telephone metadata can make people wary about expressing their free speech rights, and the government's collection of millions of phone records will raise huge temptations to use the information for abusive purposes, said David Medine, chairman of the PCLOB.

It's important for U.S. residents to continue their "healthy skepticism" of government bulk collection of personal data, he said.

The collection of so much data diverts the attention of investigators from real terrorism plots, said Michelle Richardson, legislative counsel with the American Civil Liberties Union. In April's Boston Marathon bombings and other recent attacks, law enforcement agencies had tips about the suspects but didn't act on them, she said.

Investigators should connect the dots they have, Richardson said. "Our government is obsessed with collecting more information instead of using the actionable information it has," she added.

Grant Gross covers technology and telecom policy in the U.S. government for The IDG News Service. Follow Grant on Twitter at GrantGross. Grant's email address is grant_gross@idg.com.